1. Field of the Invention
This invention relates to rubber compositions particularly for use in automobile tire treads.
2. Prior Art
To cope with the advent of expressway networks or sophisticated automobiles, tire grip on dry or wet roads is a great concern from the safe driving point of view. Such tire quality is closely associated with frictional resistance between the tread and the road. The tread rubber should to this end be highly resistant to friction against the road surface, which resistance is dependent upon the magnitude of hysteresis loss due to the tread being periodically deformed while in high-speed, frictional contact with irregularities on the road. Hysteresis loss is known to be fairly correlative to a certain loss factor (tan .delta.) determined by dynamic viscoelasticity at a temperature of about 0.degree. C. in a frequency band of 10 to 20 Hz. To improve tire grip, therefore, involves the use of a tread rubber having increased tan 67 in the vicinity of 0.degree. C.
Fuel economy is another yet important tire quality with respect to the resource- and energy-saving requirements. This quality, generally dominated by rolling friction of the tire, has been found obtainable with a tread rubber being rendered small in respect of hysteresis loss, hence loss factor, and eventually rolling resistance at from 50.degree. to 70.degree. C. at which the tire is run.
To attain enhanced tire grip and fuel saving characteristics, automobile tire manufacturers have made great efforts to develop an improved rubber material capable of providing a good balance of loss factors, higher tan .delta. at about 0.degree. C. and lower tan .delta. at from 50.degree. to 70.degree. C., at from 10 to 30 Hz.
Vinyl-rich polybutadiene rubber (V-BR) has been proposed which is abundant in 1,2-bond content, generally more than 40%, as taught for instance in Japanese Patent Laid-Open Publication No. 55-12133 and No. 56-127650. V-BR, though acceptable in wet skid resistance and also in rolling resistance, is readily susceptible to abrasion and destruction.
Two types of vinyl-rich styrene-butadiene rubber (V-SBR) are disclosed in Japanese Patent Laid-Open Publication No. 57-55204 and No. 57-73030. One such rubber has a 1,2-bond content of 42 to 70% and a styrene content of 10 to 30%, whereas the other is 60 to 95% in 1,2-bond content and 3 to 30% in styrene content. The first mentioned V-SBR excels in mechanical strength compared to V-BR, but suffers from insufficient tire grip and inadequate rolling resistance. While the last mentioned V-SBR, because of is high glass transition temperature, is effective in improving tire grip, it fails to fully produce such beneficial effect when blended with natural rubber (NR) or polyisoprene rubber (IR). This is because too much 1,2-bond renders the rubber blend compatible.